1. Field of the Invention
The invention relates to an auto gain control circuit. In particular, the invention relates to an auto gain control circuit with zero decibel gain.
2. Description of the Related Art
FIG. 1 illustrates a conventional auto gain control circuit 100 with zero decibel gain. Auto gain control circuit 100 comprises peak detector 110, variable gain amplifier 120, multiplexer 150, comparator 164 and integrator 162. Output terminal is controlled by control signal S0 to output signals S11 or S12. In the application of zero decibel gain, multiplexer 150 directly outputs signal S11. In the application of auto control gain, multiplexer 150 outputs signal S12. Peak detector 110 detects peaks of signal S12 to generate peak signal S14. Comparator 164 compares peak signal S14 with reference signal Vref to generate comparative signal S15. When peak signal S14 is higher than reference signal Vref, comparator 164 outputs comparative signal S15 to decrease charges of integrator 162 for decreasing the voltage level of control signal S16. Similarly, when peak signal S14 is lower than reference signal Vref, comparator 164 outputs comparative signal S15 to increase charges of integrator 162 such that the voltage level of control signal S16 increases. Thus, the output range of output signal S12 is adjusted by variable gain amplifier which is controlled by control signal S16. When this negative feedback loop locked, the output signal strength will be stable.
However, noise generated by multiplexer 150 is directly transmitted to output terminal during switching of multiplexer 150, interfering with output signals. If the instant variation of the output signals is too large, subsequent stage circuits will be saturated or abnormal. If conventional auto gain control circuit 100 is applied in audio speaker systems, during switching of multiplexer 150, users will be disturb by excess noise generated by outputting speakers.